There are numerous applications for which the input power must come from a DC voltage source. However, in many instances the available DC voltage supply has a voltage greater than is required or can be tolerated by an application. In these instances, a DC to DC step-down voltage converter is generally needed. Additionally, many applications can only tolerate much lower voltages while they are warming-up. In these instances, the voltage supplied to the application must be lower during start up. This can be achieved through some sort of start up mechanism. One example of an industry that will require such an application is the automotive industry. As the automotive industry moves from a 12 volt standard to a 42 volt standard, many automotive applications will be in great need of a DC voltage converter and start up mechanism.
Generally, most automobiles get their electrical power from a 12 volt battery. However, as the demand for more and more features in motorized vehicles continues to grow, so do the power demands on the vehicle""s power source. Features such as electrically heated seats, electrically controlled valves and electrically-assisted power steering all place a great power demand on a vehicle. Eventually, to accommodate a greater array of features, the current 12 volt battery will be replaced by a 42 volt battery. Although there are some systems that will benefit from a new 42 volt standard power supply, some, such as incandescent lighting, actually operate better at 12 volts.
One possible method of adapting headlights to a 42 volt battery would be to design new headlights that can run directly off the 42 volt battery. However, incandescent light bulbs, such as the bulbs used in automotive headlamps, operate at a given power level. This means that if the voltage of the power supply feeding the incandescent light bulb increases, the current must decrease in order to maintain the required power. In order to achieve a decrease in current, the filament of the bulb must get longer and thinner to increase the resistance. This would require a filament made from a material that can be made very thin and still withstand shock and vibration. Therefore, running the headlamps directly off the 42 volt power supply would not allow the use of existing headlamps and would require the use of new more resilient filaments.
Alternatively, it has been suggested that the headlights may be run off the 42 volt battery if the voltage is converted from a purely DC voltage to a pulsed output. This solution is based on the fact that a pulsed signal has an effective or root-mean-square (xe2x80x9crmsxe2x80x9d) voltage that is much lower than that of a purely DC voltage with the same amplitude. Additionally, in general, the length of time for which the filament can withstand 42 volt pulses increases as the pulses become shorter. However, this solution has a problem involving the warm-up period of the headlight. Headlights that include filaments generally need a period to warm-up. When these headlights are cold, their resistance is very low. As the filament warms up, the resistance of the filament increases which causes the current through the filament to decrease. When a voltage pulse is inputted into a cold headlight, the filament in the headlight experiences a large surge of current which can damage the headlight. This danger is increased as the magnitude of the voltage pulse increases. Therefore, the amplitude of the output of the battery needs to be reduced during the time it takes for the filament to warm-up. This would allow time for the resistance of the filament to increase, thus allowing the input current to decrease before the filament is subjected to the full voltage. However, decreasing the amplitude of the output of the battery during warm-up would require relatively complex and expensive analog circuitry.
Other solutions have included replacing the filament-type bulbs with high-intensity discharge lamps because these lamps need higher voltage to operate anyway. However, this would require replacing the filament-type bulbs already existing in automobiles and the use of complex circuitry to control the discharge current of the high-intensity discharge lamps (see U.S. Pat. No. 5,083,065). Additionally, this solution does not address the problem of current surge during warm-up.
Due to a desire to continue using existing incandescent bulbs or filament-type bulbs with a new 42 volt power supply, an intermediate solution has been proposed. During a transition period, automobiles will include both a 36 volt and a 12 volt power supply. Although this does increase the power available to the electrical and electronic applications of the vehicle and allows the use of existing filament-type bulbs, it adds a layer of complexity and duplication to the vehicle""s power system, primarily due to the additional battery that is needed. Additionally, the added complexity adds weight to the vehicle and the transition period delays the ultimate adoption of the 42 volt power supply.
In order to continue the use of existing incandescent bulbs while avoiding the use of an intermediate solution, some sort of step-down DC to DC conversion and some sort of soft start mechanism must be supplied. Pulse-width modulation (xe2x80x9cPWMxe2x80x9d) has been used to decrease the voltage to incandescent bulbs used as headlights in automobiles with the standard 12 volt power supply. The decrease in voltage is used to decrease the output illumination to provide daytime running lamps (xe2x80x9cDRLsxe2x80x9d) or to enable the use of the high beams when the low beams have burned out. However, the use of PWM requires rather complex circuitry for implementation because the on-time of each output voltage pulse must be modulated. Furthermore, PWM does not address the issue of the power surge to the bulb before warm-up. Therefore, additional complex circuitry is needed to form the leading edge of the pulses so that a gradual current increase can be achieved until the bulb has warmed-up.
It is an object of the present invention to provide a DC to DC step-down converter and a soft start mechanism using dual modulation. It is also an object of the present invention to provide a DC to DC step-down converter and a soft start mechanism that will work with most DC applications and use relatively simple circuitry. It is also an object of this invention to provide a DC to DC step-down converter to enable the use of existing filament bulbs as headlights in automobiles with a power source of 42 volts that does not include the disadvantages of previously suggested solutions. Other objects and advantages of the present invention will be apparent from the following summary and detailed description of the preferred embodiments.
In accordance with the teachings of the present invention, an apparatus and method are provided for DC output regulation using dual pulse modulation. A DC output is regulated by reducing the rms value of the output voltage by twice modulating the output. The DC voltage is first mixed with a lower frequency signal and then with a higher frequency signal in order to reduce the voltage seen by a load powered by the DC output. The rms value of the voltage can be adjusted by increasing or decreasing the number of high frequency pulses that are mixed with the on-time of the lower frequency pulses. This can be used to provide a soft start to a load during its warm-up period.
DC output regulators using dual pulse modulation can be used to drive the various lights present in a vehicle, such as the headlights. In general, a higher frequency pulse generator and a lower frequency pulse generator produce signals that modulate the voltage from the battery of the vehicle. If the battery is a 42 volt battery, this apparatus and technique allow the use of preexisting filament-type lights and headlights, without degrading their lifespan.
The invention provides numerous embodiments which will be understood by those skilled in the art based on the present disclosure. Some of these are described below and are represented in the drawings by means of several figures, in which: